The mutations delta F508, R553X, and G542X account for 75 percent to 80 percent of defective CFTR alleles among Caucasians in the United States. We are investigating new approaches for understanding and/or recovering function in these clinically important CFTR mutations. In recombinant LLC-PK1 cells, we have developed an effective in vitro strategy for correcting the delta F508 processing defect by treating these epithelial cells with low concentrations of a differentiating agent (2 percent dimethyl sulfoxide x 4 days). This treatment markedly increases the processing of delta F508 CFTR to the plasma membrane and represents one of the first opportunities to study the role of delta F508 CFTR in C1- secretion across a cell monolayer. Glycerol, butyrate and temperature modulation do not augment delta F508 processing in any cell monolayer, and had no effects on LLC-PK1 cells in our experiments. We intend to clarify the cellular mechanisms by which the delta F508 processing is overcome in this model. We will also test the relevance of these findings to human airway epithelial cells and delta F508 CFTR mice. In another series of experiments, we have shown that CFTR truncated at position 553 and 542 can activate C1- permeability in epithelial cells. We provide evidence that these truncated CFTR proteins retain the ability to modulate amiloride-sensitive Na+ transport in vivo. This is the first example of regulation of an epithelial Na+ channel by a short CFTR polypeptide (transmembrane segment 1, plus a portion of the first nucleotide binding domain). We have designed experiments to test these two CFTR truncation mutations (as well as their constituent NBD-1) in vitro and in vivo for effects on epithelial Na+ and C1- transport.